Man, this question always gets me fired up because it’s so central to everything frustrating and brilliant about Dumbledore. I don't think he 'ignores' Harry so much as he's orchestrating a long game that no one else, especially a teenager, could possibly see. Look at the end of 'Order of the Phoenix'—Harry is destroying the headmaster’s office, screaming about being possessed, and Dumbledore is just sitting there, taking it. That felt like the coldest shoulder imaginable when I first read it. But on re-reads, you realize Dumbledore knew Voldemort was using that connection and any overt comfort or guidance would have tipped their hand. He was trying to protect the secret of the horcruxes, yeah, but he was also protecting Harry from the burden of that knowledge too soon.
Still, the emotional cost was massive. Harry spent that whole year feeling abandoned, and Dumbledore let him. That’s the part that’s hard to swallow—the calculation that Harry’s loneliness and anger were acceptable casualties. It’s like Dumbledore saw him more as a piece in a war, the 'chosen one,' before he saw him as a kid who’d lost his godfather. Maybe that’s the tragedy of leadership; you make the ruthless choice for the greater good, and you live with the person you become because of it. Dumbledore’s avoidance after Sirius died wasn’t just strategy, it was shame. He couldn’t look Harry in the eye because he felt responsible for putting Sirius in harm’ s way.
In the end, his distance was a terrible, necessary kind of care. He had to harden Harry, to let him feel isolated, so that when the time came, Harry would be capable of walking into the forest alone. Doesn’t make it any less painful to read through, though. The silence between them after the Department of Mystodies is heavier than any spell.